1. Field of the Invention
The present invention relates to a switching mode power supply circuit (SMPS) using repetitive switching of the states which ON and OFF of a transistor to cause a direct current being applied to a primary coil of transformer to induce a current into a secondary coil, and more particularly, to a protection circuit for a switching mode power supply circuit using the ON and OFF operation of a transistor to allow an interruption when an over voltage is outputted.
2. Discussion of the Prior Art
As shown in FIG. 1, a protection circuit for a switching mode power supply circuit is constructed such that a power input terminal V.sub.IN is connected to a collector of a transistor TR.sub.1 switching ON and OFF a current flowing through a primary coil T.sub.11 of a transformer T.sub.1 ; an emitter of the transistor TR.sub.1 is connected to an intermediate terminal of a secondary coil T.sub.12 of transformer T.sub.1 ; the end terminal of the secondary coil T.sub.12 are connected to a base of the transistor TR.sub.1 through a resistor R.sub.14, a capacitor C.sub.1 and a feed back circuit 1; secondary coils T.sub.13 T.sub.14 are, respectively, connected to each input terminal of a rectifier 2 which is composed of a diode D.sub.1, capacitors C.sub.2 C.sub.3 and a coil L.sub.1 and another rectifier 3 which is composed of a diode D.sub.2, capacitors C.sub.4 C.sub.5 and a regulator REG. On the other hand, an output terminal of the rectifier 2 is connected to a zenor diode ZD.sub.1 and a resistor R.sub.2 through a resistor R.sub.1. Resistor R.sub.1 is also connected to an emitter of the transistor TR.sub.2 through the resistor R.sub.3 and to an inversion input terminal (-) of a comparator COMP through a resistor R.sub.4. The output terminal of rectifier 2 is also connected to a resistor R.sub.6 and to a non-inversion input terminal (+) of the comparator COMP through a resistor R.sub.5. The output terminal of comparator COMP is connected to a base of the transistor TR.sub.2 through a resistor R.sub.8. A collector of transistor TR.sub.2 is connected to a base of a transistor TR.sub.3 through a resistor R.sub.10. An emitter of the transistor TR.sub.3 is connected to a gate of a thyristor SCR.sub.1 and a capacitor C.sub.6 through a resistor R.sub.13. An anode of thyristor SCR.sub.1 is connected the diode D.sub.1, capacitor C.sub.2, and coil L.sub.1.
The operation of the conventional circuit will be explained below.
When a direct current is inputted to a power input terminal V.sub.IN, the direct current power is applied to a primary coil T.sub.11 of transformer T.sub.1, thereby inducing a current in the secondary coils T.sub.12 and T.sub.13 in accordance with the ON and OFF switching of the transistor TR.sub.1. The power induced in a secondary coil T.sub.12 is applied to a base of transistor TR.sub.1 through the feed back circuit 1, thereby causing the ON and OFF switching of the transistor TR.sub.1 to be repeated.
At this moment, the power is induced in another secondary coil T.sub.13 and is rectified via the rectifier 2 which outputs a constant voltage V.sub.1. At the same time, the outputted constant voltage V.sub.1 is applied to a zenor diode ZD.sub.1 through a resistor R.sub.1 to becomes another constant voltage. It is then applied to an inversion input terminal (-) of the comparator COMP through a resistor R.sub.4. Further, the power is induced in to another secondary coil T.sub.14 is rectified via the rectifier 3 which outputs a constant voltage V.sub.2 the output voltage V.sub.2 is applied to a non-inversion terminal (+) of the comparator COMP through a resistor R.sub.5.
If the voltage which applied to the inversion input terminal (-) of the comparator COMP is lower than the voltage of non-inversion input terminal (+) of the comparator COMP, a high potential signal is outputted from the comparator COMP and the transistor TR.sub.2 is turned OFF, accordingly the transistor TR.sub.3 also turns OFF. In response to this OFF state, the gate of the thyristor SCR.sub.1 is applied with a low potential, and the thyristor SCR.sub.1 is turned OFF, thereby causing the rectifier to execute normal operations.
If the voltage applied to the inversion input terminal (-) of the comparator COMP is higher than the voltage applied to the non-inversion input terminal (+) of the comparator COMP, a low potential signal is outputted from the comparator COMP, and in contrary with above description, the transistor TR.sub.2 turns ON causing the transistor TR.sub.3 to also turn ON. A high potential signal is outputted to its emitter. This high potential signal is applied to the gate of the thyristor SCR.sub.1 thereby causing the thyristor SCR.sub.1 to become conductive and rendering the operation of the rectifier 2 to be terminated.
However, in such a conventional protection circuit for switching mode power circuit, if an overvoltage is outputted from the rectifier 2, the switching transistor TR.sub.1 continuously repeats switching ON and OFF; therefore, the direct current power input to the power input terminal V.sub.IN is unnecessarily lost such that the, switching mode power supply circuit cannot be protected.